[forcefield] TriangularShellForceField: draw element showing the measured values (color map)

[EulalieCoevoet]

In this PR:

• Fixes compilation issues
• TriangularShellForceField:
  • draw elements showing the measured values, using a ColorMap from blue to red
  • otherwise draw the triangles like done in TriangularBendingFEMForceField
  • only draw if getShowForceFields is true. The previous implementation was always and only drawing arrows.

Screenshot - von Mises stress color map

Screenshot - force fields classic visu

[EulalieCoevoet]

I'm sorry, I only noticed now that it was already proposed in #10.
Although there is a requested change and nothing has been done after a year now.

I'm okay to add the missing features from #10. Personally I would only keep the showArrows option. You tell me.

Edit: I was wrong, the requested changes were made but the PR has been forgotten I guess. I have added the option to hide the arrows. In this PR, in addition to #10, I propose to draw the triangles by default if the force field flag is checked.